Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing. "Long" or "soft" x-rays (a.k.a. Extreme UV) (wavelength range of .lambda.=100 to 200 .ANG.) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection lens onto a wafer. Reticles for EUV projection lithography typically comprise a silicon substrate coated with an x-ray reflective material and an optical pattern fabricated from an x-ray absorbing material that is formed on the reflective material. In operation, EUV radiation from the condenser is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the x-ray absorbing material. The reflected radiation effectively transcribes the pattern from the reticle to the wafer positioned downstream from the reticle. A scanning exposure device uses simultaneous motion of the reticle and wafer, with each substrate being mounted on a chuck that is attached to an X-Y stage platen, to continuously project a portion of the reticle onto the wafer through a projection optics. Scanning, as opposed to exposure of the entire reticle at once, allows for the projection of reticle patterns that exceed in size that of the image field of the projection lens. Mirrors are mounted along the sides of a stage platen; and interferometer heads that direct laser beams onto the associated mirrors and detect the beam reflection therefrom are employed for position measuring purposes. Movement of the platen stage is accomplished with motorized positioning devices. A stage platen similarly supports the wafer substrate.
Prior art stage platen typically suffer from a significant drawback in that the sensitivity of measurement accuracy of the stage platen position is adversely affected by temperature. The electromagnetic motors which drive the elements of the stage platen relative to one another are a significant heat source adversely affecting the performance of the laser interferometry typically used to determine the actual stage platen position.
Additionally, prior art stage platens suffer from reduced performance due to their relatively high mass which reduces the stage mechanical resonance frequency and thereby lowers the stage platen performance. If the stage platen is made stiffer to compensate, this may add even more mass. Furthermore, higher mass requires more motor power, which presents more potential for heating.
Therefore, there is a significant problem in the prior art of impeded stage platen performance in terms of accuracy and speed caused by the relatively high weight of the stage platen and the heat generated by the stage platen movement. A stage platen with high mass and low stiffness cannot be controlled to the accuracy needed in state-of-the-art photolithography systems. The art is in search of improved stage platen that can be precisely maneuvered and which ultimately result in the enhanced quality of the printed wafers fabricated.